An industrial cable insulation tester

Abstract

The chemical industry monitors the acidity and alkalinity of aqueous solutions. The high impedance pH and redox electrochemical cells used for monitoring output a weak signal dependant on the hydrogen ion concentration over high performance, low noise coaxial cables. These cables are a critical part of the metering system, sometimes carrying these delicate signals over distances of 20m. The independent performance of these cables must therefore be tested and verified before shipping to the end user. The development of this project sought to explore different insulation resistance measurement techniques such that a high accuracy and precision reading may be achieved. The final design consisted of a multi-channel cable test rig with the capability of repeatable readings of insulation leakage currents on the order of tens of picoamps. Furthermore, a variety of cables connector assemblies can be tested to check the continuity of the cable by means of a loop-back test. The sensitive current measuring analogue circuit is prone to error caused by moisture, dirt, noise and low resistance insulators which required careful board layout and routing techniques as well as environmental and electromagnetic shielding. Analogue to digital conversion is performed using a 12-bit ADC and processed by a 16-bit microcontroller. Test results are then displayed on an LCD and LEDs to the operator in order to achieve a clean and simple user interface. The final system is to be integrated into the production line for pH probe cables for ProMinent Fluid Controls Ltd.